The present invention generally relates to the repair of turbine rotor wheels and particularly the dovetails used for securing buckets or blades to the rotor wheels. The invention is particularly applicable to the repair of both steam turbine and gas turbine rotors.
Steam and gas turbines typically used for power generation are comprised of multiple stages each defined by alternating rows of stationary nozzle vanes and turbine buckets or blades mounted on wheels fixed to the turbine rotor. The buckets are conventionally attached to the wheels by a dovetail connection. A number of different types of dovetails may be employed. For example, in finger-type dovetails that enable radial loading of buckets onto the wheel, the outer periphery of the rotor wheel has a plurality of axially-spaced circumferentially-extending, stepped grooves for receiving complementary fingers on each of the bucket dovetails. After the buckets are stacked about the rotor wheel, pins are typically passed through aligned openings in the fingers of each of the bucket dovetails and the groove walls in the wheel to secure the buckets to the wheel.
Other types of dovetails (male or female) have generally pine-tree shapes, which enable tangential or axial loading of the buckets, depending on the orientation of the dovetails.
In any event, the dovetail connections between the buckets and wheels are highly stressed and, after years of operation, may wear out and/or crack. On low pressure steam turbine rotors, cracking occurs typically as a result of stress corrosion. In high pressure steam turbine rotors, cracking typically occurs as a result of creep rupture and/or low cycle fatigue. It will be appreciated that the magnitude of the stresses in the rotor wheel are very substantial at the radially outward location of the wheel dovetail because of stress concentration factors developed by the dovetail geometry. That is, peak stresses are significantly higher in the wheel dovetail as compared with locations radially inwardly of dovetails. For example, the pin openings in the finger-type dovetail and the machined areas of the wheel defining the fingers concentrate the stresses in the dovetail area and, over time, may cause cracking as a result of one or more of the aforementioned failure mechanisms.
In the past, the utility operator, upon inspection of the rotor and identification of a significant crack in one or more of the turbine wheels, particularly at the dovetail connections had essentially two choices: the entire rotor could be replaced, or the damaged rotor wheel could be repaired by employing a conventional weld buildup process. The first option is costly and may involve considerable downtime before a new rotor is available for installation. For that reason, removal of the damaged dovetail from the rotor wheel and replacement of the removed dovetail with built-up weld material has been the principal repair method of choice.
In a typical weld buildup process, the rotor is first removed from the turbine and the buckets are removed from the damaged wheel. The damaged dovetail portion of the wheel is then removed, leaving a blank outer rim. As used herein, “dovetail” in the context of what is removed and replaced, refers to an outer portion of the wheel that incorporates all of the individual bucket dovetails, unless otherwise noted.
Weld material is applied to the rim in multiple passes to provide a weld build-up, which can later be machined to provide replacement dovetails. The weld material can be the same as or different from the material from which the rotor wheel is made. For example, in U.S. Pat. No. 4,940,390, a GTAW process is used to deposit a weld metal of 12 Cr material onto a Ni—Cr—Mo—V rim. 12 Cr material is much more resistant to stress corrosion cracking than the Ni—Cr—Mo—V material. However, welding processes in general are prone to defects such as porosity and slag inclusions in the weld metal, and it is difficult to optimize the properties of the weld material when it is being deposited on the wheel rim.
There are also specific limitations on the buildup of weld material on a wheel, which render turbine rotor wheel build-ups as a method of repair only marginally satisfactory as explained in commonly-owned U.S. Pat. No. 6,049,979.
In the '979 patent, there is described a dovetail repair process for a turbine rotor which, instead of a weld build-up repair, provides a ring replacement, typically a forging, for the entirety of the damaged dovetail. The forged replacement ring, provided in segmented form or as a full 360° annular ring, can be formed of the same or improved materials in comparison with the original rotor so as to provide improved resistance to the various failure mechanisms noted above. The replacement ring is beneficially virtually free of defects, which might otherwise be extant in repaired dovetail characterized by a weld build-up of the same or similar material. Also, the material of the replacement ring is not a function of the welding process or the weld material employed to secure the ring to the rim of the wheel body after the damaged dovetail has been removed. Consequently, the forged replacement ring can be formed of materials which provide optimum properties for resistance to one or more of the different types of failure mechanisms. For example, for rotors formed of Ni—Cr—Mo—V or Cr—Mo—V or 12 Cr, a 12 Cr material such as 12% CrCb or an Inconel-based material can be employed. The weld material can be any weld material, which is compatible with both the base material and the forged replacement ring material, for example, a 12 Cr—Ni—Mo. The nature of the weld material is less significant to the welding process employed in this case because the weld is formed along a relatively low stress area of the wheel and the only purpose of the weld is to join the wheel and the ring. That is, the weld material is not required to be as resistant to high stresses as is the dovetail per se.
As further described in the '979 patent, the substantially defect-free replacement forged ring can be welded to the rim of the wheel body by a FineLine™ welding technique. Characteristic of the FineLine™ welding technique is the provision of a substantially linearly extending non-beveled extremely narrow groove which does not introduce significant porosity or slag into the weld material and which is relatively unsusceptible to welding problems with welding materials usually considered difficult to weld. By using the FineLine™ welding technique and locating the weld in a low stress region of the wheel, the forging can be formed of the most optimal material and the material can be selected to increase the strength and damage-resistant properties, e.g., by using forgings of relatively high carbon, columbium and other favorable materials.
In terms of repairing a damaged dovetail, once it has been determined that the rotor is in need of repair, the rotor is lifted from the turbine and placed on a horizontal axis upon bearings enabling motorized rotation of the rotor, i.e., the damaged rotor is placed in a lathe. The buckets on the rotor wheel requiring a dovetail repair are removed, for example, by removing the pins of the finger-type dovetail, the buckets thereby being removable in a radial outward direction. The damaged dovetail is then removed from the wheel by a machine tool operation, leaving a wheel body having a reduced diameter rim. The annular cut through the wheel to remove the damaged dovetail is made at a relatively low stress location about the wheel, i.e., radially inwardly of the dovetail. Weld preps are provided on both the wheel rim and the I.D. of a replacement ring. The ring, of course, is previously fabricated of the desired material and sized for the wheel body on which it will form the dovetail replacement. The ring is preferably formed either in multiple sections, for example, two 180° sections, or as a 360° ring and is applied about the rim of the rotor and temporarily secured, for example, by tack welding or bolting.
Once the welding is complete, and in the case of a segmented ring, the ends of the ring sections are butt-welded to one another. Upon completion of the butt-weld, the blocks are cut from the ring and the butt-welding head is removed.
A stress relief is then performed with the rotor oriented horizontally or vertically, depending on available equipment. Induction coils (or other suitable means) provide a heat treatment to the ring and rotor at selected elevated temperatures, thereby stress-relieving the weld. The rotor is then cooled down at a controlled rate by controlling the heat applied to the rotor. A final ultrasonic inspection of the cold rotor is then effected. Assuming the welds are without defect, the cutting machine is next employed to form the dovetails in the forged ring now welded to the rotor wheel body. In a finger-type dovetail, a milling head is secured to the machine and cuts the fingers in the outer surface of the ring. After forming the dovetail, the annular welds are typically shot-peened to introduce compressive stresses along the outer surfaces of the welds. These compressive stresses increase the resistance of the material to stress corrosion cracking. Subsequent to shot-peening, the buckets are disposed in the new dovetails and the rotor is balanced. The repaired rotor is then placed back into service.
Even with the advantages of the weld repair processes described in the '979 patent, however, a need has developed for even greater weld efficiencies as well as greater utilization of opportunities provided by the use of replacement rings on rotor wheels in terms of accommodating new dovetail designs that may be incorporated into the repair process.